A typical train includes one or more locomotives pulling a plurality of load cars. Each vehicle in the train includes a plurality of steel wheels that roll along the metal rail as the train is propelled along the track. Proper interaction between the wheel and the rail is critical for safe, reliable, efficient operation of the train.
A rail includes a bottom mounting flange, a top railhead that makes contact with the rail vehicle wheel, and a flange interconnecting the flange and the railhead. A rail vehicle wheel includes a center hub mounted onto the vehicle axle, a plate extending outwardly from the hub, and an outer rim surrounding the plate for making contact with the rail. A rail vehicle wheel set includes two wheels connected by their respective hubs to opposite ends of an axle. The rim includes an outside diameter tread that may be flat or tapered and a flange extending outwardly from a back side of the tread. The tread rides along a top surface of the railhead for supporting the vertical weight of the vehicle. The flange extends along and makes contact with a side of the railhead for providing lateral support to allow the wheel to follow along the path of the railhead. Flanges are provided on only one side of each wheel along an inside of the rail.
Rail vehicle wheels suffer wear over time due to their contact with the rail. The treads wear as a result of their contact with the top of the rail, particularly in the event of the wheel slipping with respect to the rail during acceleration or braking events. The wheel flanges will wear due to their contact with the inside surface of the railhead, particularly on curves and through switches. Consequently, rail wheel wear must be monitored to ensure that dimensions of the wheel subject to wear are sufficient for continued safe use.
Rail wheel dimensions, such as rim thickness, flange thickness, flange height, and a rim diameter of a train wheel, are subject to Federal Railway Administration (FRA) and/or railroad dimension limits. Such dimension limits may be based on wheel set membership in a truck and/or in a rail vehicle, and whether or not an axle of the wheel set has been shimmed. When a wheel dimension wears to a value beyond a dimension limit, the wheel must be machined to an acceptable dimension and/or axle shims installed to bring the wheel back into compliance with the dimension requirements. If there is not enough material left on wheel to support further machining, or shims cannot be used to extend the life of a wheel set, the wheel set must be condemned. In addition, wheel dimensions must be checked with respect to dimensions of other wheels of the same truck and other wheels of other trucks of the rail vehicle to insure that dimension differences among the wheels do not exceed predetermined difference limits. For example, FRA wheel truing rules may require that there be no more than a 0.75 inch difference among rim diameters for wheels of a truck and no more than a 1.25 inch difference among rim diameters for all the wheels of a locomotive. Furthermore, the presence of shims may change wheel dimension limit and wheel dimension difference limit requirements.